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Friday, February 13th 2015

11:30 am:

Two-dimensional AKLT model on a honeycomb lattice has been shown to be a universal resource for quantum computation. In this valence bond solid, however, the spin interactions involve higher powers of the Heisenberg coupling (S_i *S_j)^n, making these states seemingly unrealistic on bipartite lattices,

where one expects a simple antiferromagnetic order. We show that those interactions can be generated by orbital physics in multiorbital Mott insulators. We focus on t_{2g} electrons on the honeycomb lattice and propose a physical realization of the spin-3/2 AKLT state. We find a phase transition from the AKLT to the Neel state on increasing Hund's rule coupling, which is confirmed by density matrix renormalization group (DMRG) simulations. An experimental signature of the AKLT state consists of protected, free spins-1/2 on lattice vacancies, which may be detected in the spin susceptibility.

Friday, April 3rd 2015

2:00 pm:

We provide a solution for the supersymmetric CP(N-1) at large N

written in superfields. Specifically, we find the Kahler potential of

the model by supersymmetrizing the known results for the effective

potential of the theory. We also generalize the problem by introducing

twisted masses, and by breaking supersymmetry to N=(0,2). Such

theories arise as effective models on the heterotic vortex strings.

Monday, April 27th 2015

2:00 pm:

Wednesday, September 9th 2015

2:00 pm:

We consider a Dark Matter scenario in a singlet extension model of the Minimal Supersymmetric

Standard Model, which is known as nMSSM. We find that with high-scale supersymmetry breaking,

the singlino can obtain a sizable radiative correction to the mass. This opens a

Dark Matter scenario with resonant annihilation via the exchange of the Higgs boson.

We show that the current Dark Matter abundance and the Higgs boson mass can be

explained simultaneously. This scenario can be probed by XENON1T.

We also mention the possibility of Electroweak Baryogenesis at high temperature in this model.

If there exist vector like matters coupled to the singlet multiplet, the thermal effects deform

the Higgs potential at high temperature which derives a first order phase transition.

We show that a strong first order phase transition can occur when the temperature is around

the supersymmetry breaking scale, which can be TeV scale.

This talk is based on

K. Ishikawa, T. Kitahara and M. Takimoto,

``Singlino Resonant Dark Matter and 125 GeV Higgs Boson in High-Scale Supersymmetry,''

Phys. Rev. Lett. 113, no. 13, 131801 (2014)

[arXiv:1405.7371 [hep-ph]].

and

K. Ishikawa, T. Kitahara and M. Takimoto,

``Towards a Scale Free Electroweak Baryogenesis,''

Phys. Rev. D 91, no. 5, 055004 (2015)

[arXiv:1410.5432 [hep-ph]].

Thursday, September 10th 2015

7:00 pm:

I will talk about the search for a unified theory of the laws of physics including quantum mechanics, which governs the very small, and general relativity, which governs the very large. Stephen Hawking showed 40 years ago that these theories make conflicting predictions near black holes. This ignited a battle that continues to this day: either quantum mechanics must break down, or our understanding of spacetime must be wrong. The latest wrinkle is the `firewallâ€™ paradox: if quantum mechanics is to be saved, then an astronaut falling into a black hole will have an experience very different from what Einsteinâ€™s theory predicts. This has led to many new ideas that may lead to the unification of these two great theories.

Friday, September 11th 2015

2:00 pm:

Statistical mechanics depends on chaos --- the sensitive dependence on initial conditions --- to produce ergodic mixing. It has been known for more than four decades that black holes satisfy thermodynamic laws, but the associated chaotic behavior has been discussed only recently. I review these developments, and extend them from eternal black holes to black holes that form and decay.

Thursday, October 8th 2015

12:30 pm:

I present a simple extension of the Standard Model that gives rise to post-sphaleron baryogenesis by introducing colored scalar fields. The model can accommodate a fermionic dark matter (DM) candidate of O(GeV) mass whose stability is tied to proton stability. The supersymmetric extension of this model is straightforward and can realize a multicomponent DM scenario. I discuss prospects for direct and indirect detection of the DM candidate(s) and possible collider signals of the model.

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